An ice disc 55 feet across has been spotted swirling round and round atop the water of North Dakota's Sheyenne River.

According to the Associated Press, record-setting air pressure combined with subfreezing temperatures to turn the river water to ice – but not all at once. When the first chunks of floating ice took shape, they got caught in the river's eddy and started to spin in a circle, collecting into the disc seen here. Here's a video, shot in the UK in 2010, that appears to show one such disc coming into being:

"It's not a continuous sheet of ice," said Allen Schlag, a National Weather Service hydrologist in Bismarck, ND. "If you were to throw a grapefruit-size rock on it, it would go through. It's not a solid piece of ice — it's a collection of ice cubes."